Selling to CarMax

I recently took my Jeep Patriot to CarMax and had it appraised and was offered what I think is a good deal. I was planning to take it back within the allotted week and accept the offer but hadn't done so yet as I do not have a replacement vehicle.
Yesterday, I backed into my sister-in-law's vehicle, denting my back bumper. As this was all in the family, no insurance or police was involved and we will settle on our own.
If I take my car back to CarMax and have it re-appraised, any guesses as to how much I will lose off the initial offer? The initial offer was $4,000. The body of the car was not perfect to begin with as there are scratches and even dents here and there. Will they see this as just another imperfection in the body and dock me the several hundred it would cost them to repair or will they see it as an "accident" that will devalue the vehicle a couple thousand even though it will not show up on a Carfax or anywhere else?
I also considered taking it to a body shop and having it repaired, then taking it back to CarMax and accepting their initial offer. I would then see no reason to bring up the fender-bender.
Any thoughts from those in the know would be very helpful.
Comments
Fix it!
Car Max will probably assume the worst and dock the car more than it will cost you to have it repaired.
There are some mobile guys that can fix it at your home or in their shop such as SAMEDAY that are usually more reasonable than a body shop.
And, no, I wouldn't say anything to Car Max. If they do a good job, it'll look like new.
Thanks for the feedback. I'll check at a dent removal place and see what they could fix it for.